Radio communication networks were originally developed primarily to provide voice services over circuit-switched networks. The introduction of packet-switched bearers in, for example, the so-called 2.5G and 3G networks enabled network operators to provide data services as well as voice services. Eventually, network architectures will likely evolve toward all Internet Protocol (IP) networks which provide both voice and data services. However, network operators have a substantial investment in existing infrastructures and would, therefore, typically prefer to migrate gradually to all IP network architectures in order to allow them to extract sufficient value from their investment in existing infrastructures. Also to provide the capabilities needed to support next generation radio communication applications, while at the same time using legacy infrastructure, network operators could deploy hybrid networks wherein a next generation radio communication system is overlaid onto an existing circuit-switched or packet-switched network as a first step in the transition to an all IP-based network. Alternatively, a radio communication system can evolve from one generation to the next while still providing backward compatibility for legacy equipment.
One example of such an evolved network is based upon the Universal Mobile Telephone System (UMTS) which is an existing third generation (3G) radio communication system that is evolving into High Speed Packet Access (HSPA) technology. Yet another alternative is the introduction of a new air interface technology in Evolution UMTS Terrestrial Radio Access Network (E-UTRAN), wherein Orthogonal Frequency Division Multiple Access (OFDMA) technology is used in the downlink and single carrier frequency division multiple access (SC-FDMA) in the uplink. In both uplink and downlink the data transmission is split into several sub-streams, where each sub-stream is modulated on a separate sub-carrier. Hence in OFDMA based systems, the available bandwidth is sub-divided into several resource blocks (RB) as defined, for example, in Third Generation Partnership Project (3GPP) TR 25.814: “Physical Layer Aspects for Evolved UTRA”. According to this document, a resource block is defined in both time and frequency. A physical resource block size is 180 KHz and 1 time slot (0.5 ms) in frequency and time domains, respectively. The overall uplink and downlink transmission bandwidth in a single carrier of a Long Term Evolution (LTE) system can be as large as 20 MHz.
An E-UTRA system under single carrier operation may be deployed over a wide range of bandwidths, e.g. 1.25, 2.5, 5, 10, 15, 20 MHz, etc. As an example, a single carrier deployed over a 10 MHz bandwidth can include 50 resource blocks. For data transmission the network can allocate a variable number of resource blocks (RB) to the user equipment (UE) both in the uplink and downlink. This enables a more flexible use of the channel bandwidth. This because the channel bandwidth is allocated according to the amount of data to be transmitted, radio conditions, user equipment capability, scheduling scheme etc. In addition, the neighboring cells, even on the same carrier frequency, may be deployed over different channel bandwidths.
Multi-carrier, also known as the carrier aggregation (CA), refers to the situation where two or more component carriers (CC) are aggregated for the same user equipment. Carrier aggregation is considered for LTE-Advanced, such as Release 10 (Rel-10), in order to support wider bandwidths, i.e. bandwidths wider than 20 MHz. The use of carrier aggregation enables a manifold increase in the downlink and uplink data rate. For example, it is possible to aggregate different number of component carriers of possibly different bandwidths in the uplink (UL) and the downlink (DL).
Carrier aggregation thus allows the user equipment to simultaneously receive and transmit data over more than one carrier frequency. Each carrier frequency is generally called a component carrier. This enables a significant increase in data reception and transmission rates. For instance 2×20 MHz aggregated carriers would theoretically lead to two fold increase in data rate compared to that attained by a single 20 MHz carrier. The component carrier may be contiguous or non-contiguous. Furthermore, in case of non-contiguous carriers, they may belong to the same frequency band or to different frequency bands. This is often referred to as inter-band carrier aggregation. A hybrid carrier aggregation scheme comprising of contiguous and non-contiguous component carriers are also envisaged in LTE advanced.
In LTE advanced several contiguous and non-contiguous carrier aggregation scenarios are being considered. A scenario comprising 5 contiguous component carriers each of 20 MHz (i.e. 5×20 MHz) is considered for LTE Time Division Duplex (TDD). Similarly for LTE Frequency Division Duplex (FDD), a scenario comprising 4 contiguous component carriers each of 20 MHz, i.e. 5×20 MHz, in the downlink and 2 contiguous component carriers in the uplink is studied. It shall be understood that the number of component carriers that may be aggregated may be less than or greater than five. Thus, even more component carriers are possible to aggregate depending upon the availability of the spectrum.
In a carrier aggregation system (CA system) one of the component carriers in DL and in UL is designated as the primary carrier or primary CC (PCC), which is also termed as anchor carrier. The remaining CCs are termed as secondary CC (SCC). The primary carriers in the DL and UL may also belong to different bands in case of inter-band CA. The primary carriers generally carry the vital control and signaling information.
Typically the component carriers in carrier aggregation belong to the same technology, e.g. either all are of Wide Band Code Division Multiple Access (WCDMA) or LTE. However, carrier aggregation between carriers of different technologies is also possible to increase the throughput. Using carrier aggregation between carriers of different radio access technologies (RAT) is also referred to as “multi-RAT carrier aggregation” or “multi-RAT-multi-carrier system” or simply “inter-RAT carrier aggregation”. For example, the carriers from WCDMA and LTE may be aggregated. Another example is the aggregation of LTE and Code Division Multiple Access 2000 (CDMA2000) carriers. For the sake of clarity carrier aggregation within the same technology may be referred to as ‘intra-RAT’ or simply ‘single RAT’ carrier aggregation.
The network may configure one or more secondary component carriers (SCCs) for the user equipment supporting CA. Said one or more secondary component carriers may be configured using higher layer signaling, e.g. Radio Resource Control (RRC). The network may even configure such a user equipment in single carrier mode. The network may also de-configure any of the configured SCCs. The network may activate or de-activate any of the configured SCC anytime by using lower layer signaling e.g. by sending activation/deactivation command in the Medium Access Control (MAC). The user equipment is able to receive data on SCC which is activated. The user equipment saves its power by not receiving data on the deactivated SCC.
In radio communication systems various measurements are performed by the user equipment in support of a number of different network functions. Performing such measurements in new systems, such as those described above, raise various issues and challenges.